The present invention relates to the mixing of rubbers, and more specifically to the continuous mixing of rubbers.
In the rubber industry, the still very widely employed technique at the present time for mixing calls upon the use of internal mixers. This technique is carried out fundamentally batchwise; in order to produce a given amount of a rubber composition, one first of all weighs out the different components which are to constitute this composition and then introduces these components, or at least some of them, into an internal mixer, which assures the dispersing and mechanical work necessary in order to make the final mixture obtained as homogeneous as possible. The production of a composition, at times also called a mix, may require several successive passages through an internal mixer or different internal mixers, and may also utilize cylinder mixers.
All the operations mentioned above are independent operations. Continuous work, if it exists, is present only in the subsequent stage of production, at the time of the extrusion of semifinished products, such as, for instance, a tread or a sidewall, or a filler to be placed on a bead wire.
The replacement of discontinuous methods by a continuous method has been the object of extensive research and has remained a constant goal for a very long time for the conversion industries, the drawbacks of the discontinuous systems being so great. Despite this, no satisfactory method for the preparation of rubber compositions has been proposed up to the present time. A rubber composition is considered prepared if it contains all the intended components, including the vulcanization system, and if the rubber mass obtained can be used directly in an extrusion or molding step followed by a vulcanization step.
The failure of the continuous preparation methods seems to be due to the fact that the mixing of rubber is much more difficult than the mixing of other materials, such as plastics. The properties of the final composition depend, in fact, not only on the components contained in a composition but also on the manner in which the mixing has been effected, that is to say, they also depend on the mechanical work to which the mix is subjected while the mixing is effected.
The continuous mixing methods which are already known in general call upon the mixing effect of extruder screws. Numerous variants of screw extruders have been proposed--extruders with one screw, extruders with several screws in parallel or not in parallel, as well as numerous variants of screws for which a large number of shapes have been designed in order to obtain a good mixing effect and/or sufficient mechanical work.
It is known that, for a rubber mix, the dosaging of the different front base components must be very precise. This precise dosaging does not raise any particular problem when one proceeds by successive batches, that is to say, in batchwise processes. On the other hand, when it is a question of proposing continuous mixing, it is indispensable to be able to effect the necessary dosaging of the different ingredients continuously. Furthermore, in the case of powdered products, in addition to the difficulty of dosaging, there is the difficulty of transporting the products, particularly downstream of the dosaging.
Generally, dosaging units deposit the necessary ingredients at regular intervals on a belt conveyor which feeds the mixing unit or units proper. A recent attempt to make the mixing of rubber continuous is described in an article published in the "European Rubber Journal" for March, 1987, entitled: "A Tale of Continuous Development".
This continuous mixing process, already proposed in the prior art, effects the dosaging of components by weighing and then effects the introduction of different components into apparatus which effect a premixing, this being done at different places in the mixing chain. This, therefore, makes it necessary to install numerous separate apparatus, each carrying out a very precise function--weighing, conveying, premixing. It is only then that the different components can be introduced by gravity into feed wells, for instance in a mixer of the type described in patent application EP 0 490 056.
No continuous mixing process proposed in the prior art has succeeded in gaining acceptance in the rubber industry. The reasons for this are insufficient mixing action due to the difficulty of mixing the rubbers using the screw mixer technique and extremely great complexity of the dosaging of the components when it is necessary to effect it continuously. In short, the methods of continuous preparation have never yet made it possible to achieve the properties which are expected on the part of the final mix, and are even incapable of formulating the rubber composition with sufficient precision.
It is known that in order to obtain good mixing it is necessary to cause random movements of the components, that is to say, to cause a certain chaos. When this mixer is of the continuous type, such as, for instance, that of the aforesaid patent EP 0 490 056, it is necessary furthermore that the local flow be substantially constant within the mixer. This means that the more one strives to optimize the action of the mixing, the greater the risk of the appearance of instabilities in flow, which are incompatible with strict compliance with the proportions of the different components.